In an automatic power transmission for an automotive vehicle, the gear ratios in the forward and reverse drive ranges of the transmission are selected by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated by fluid pressure delivered from a hydraulic control system. In a known power transmission using these frictional unit, the forward drive clutch in particular is maintained coupled throughout the conditions in which an automatic forward drive range is established in the transmission. When forward drive clutch alone is permitted to be operative, the first or "low" gear ratio in the automatic forward drive range is established in the transmission mechanism. If the brake band is applied with the forward drive clutch maintained in the coupled condition, then a shift is made in the transmission mechanism from the first gear ratio to the second gear ratio in the automatic forward drive range. If, furthermore, the brake band is released and the high-and-reverse clutch in turn is made operative with the forward drive clutch held coupled, there is produced in the transmission mechanism an upshift from the second gear ratio to the third or "high" gear ratio in the automatic forward drive range. When, on the other hand, both of the high-and-reverse clutch and the low-and-reverse brake are made operative, a reverse drive gear position is obtained in the transmission mechanism. The high-and-reverse clutch is thus put into operation when either the third gear ratio in the automatic forward drive range or the reverse drive gear position is to be selected in the transmission mechanism.
When it is desired to apply engine braking under conditions in which a vehicle is being driven with the highest gear ratio available in the automatic forward drive range of the power transmission, the driver of the vehicle usually manipulates the transmission gear shift lever to select a manual forward drive range for making a shift from the highest gear ratio to the lower gear ratio or one of the lower gear ratios available in the manual forward drive range. If, in this instance, the selection of the manual forward drive range is made while the vehicle is producing a relatively high road speed, shifting is automatically made from the highest gear ratio in the automatic forward drive range to the higher one of the gear ratios in the manual forward drive range if there are more than one gear ratios available in the latter range. During engine braking condition thus brought about, the frictional unit such as the brake band to be actuated to effect the shifting is subjected to an increased driving troque to be transmitted from the transmission output shaft to the engine. If, therefore, the particular frictional unit is actuated by a fluid pressure which is used for actuating the frictional unit under other conditions, the fluid pressure is deficient for enabling the frictional unit to bear the driving torque imparted thereto so that the engine braking can not be effected reliably. If the fluid pressure to actuate the frictional unit is maintained at an increased level throughout the various operating conditions of the power transmission, for the purpose of avoiding such a problem, then the fluid pressure to be applied to the frictional units under conditions other than engine braking would be excessive for the load requirement for the frictional units and would cause the frictional units to cause mechanical shocks during shifting between gear ratios or selection of any of the gear positions.
To provide a solution to these problems, a known hydraulic transmission control system uses a pressure booster valve which is adapted to increase or "boost" the fluid pressure, or line pressure, to actuate the frictional unit to be put into operation when a manual forward drive range is selected in the power transmission. If, in this instance, the power transmission is of such a nature that two or more gear ratios are available in the manual forward drive and a shift can be made automatically between these gear ratios for enabling the vehicle to start with such a forward drive range or preventing the engine from overrunning, the fluid pressure applied to the frictional unit contributive to the higher one of these gear ratios is excessive for the load requirement of the frictional unit and will also cause production of mechanical shocks during shifting.
The present invention has been completed to solve these peoblems encountered in a hydraulic transmission control system using a pressure booster valve and is based on a discovery that it is only during shifting from the highest gear ratio in the automatic forward drive to the higher one of the gear ratios available in a manual forward drive range that an increased fluid pressure is required for actuating the frictional unit allocated to the lower one of the gear ratios. In an automatic power transmission using a brake band for achieving the higher one of the gear ratios in a manual forward drive range of the transmission, this is accounted for by the fact that the brake band requires an exceptionally large force for grasping the member to be braked upon when a downshift is to be made from the highest one of the gear ratios in the automatic forward drive range to the higher one of the gear ratios in the manual forward drive range.